Precambrian Research 105 (2001) 289–314 www.elsevier.com/locate/precamres Ion microprobe UPb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland–Kola Orogen, northern Fennoscandian Shield J.S. Daly a,*, V.V. Balagansky b, M.J. Timmerman a,c, M.J. Whitehouse d, K. de Jong e, P. Guise c, S. Bogdanova f, R. Gorbatschev f, D. Bridgwater g a Department of Geology, Uni6ersity College Dublin, Dublin, Ireland b Geological Institute, Kola Science Centre, Russian Academy of Sciences, Apatity, Russia c School of Earth Sciences, Uni6ersity of Leeds, Leeds, UK d Swedish Museum of Natural History, Stockholm, Sweden e Geological Sur6ey of Japan, Japan f Institute of Geology, Lund Uni6ersity, Lund, Sweden g Geological Museum, Copenhagen, Denmark Received 6 November 1999; accepted 23 December 1999 Abstract The Lapland–Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5–2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, SmNd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. m Evidently, adjacent Archaean terranes, with negative Nd signatures, contributed relatively little detritus, suggesting a basin of considerable extent. Subduction of the resulting Lapland–Kola ocean led to arc magmatism dated by the NORDSIM ion probe at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 Ga as shown by ion probe UPb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on SmNd, UPb and ArAr data. Lateral variations in deep crustal velocity and Vp/Vs ratio, together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland–Kola suture. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Fennoscandian Shield; Trans-crustal suture; UPb zircon * Corresponding author. E-mail address: [email protected] (J.S. Daly). 0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0301-9268(00)00116-9 290 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 1. Introduction have suggested correlations with the Nagssug- toqidian and Torngat orogens in Greenland and Collisional suture zones are first order disconti- Labrador. The crustal architecture of the LKO is nuities in the continental lithosphere originating now well known in outline but the relative lack of as the sites of rifting, sea floor spreading and later modern structural, petrological and geochrono- subduction. Criteria for their recognition include logical studies leaves room for new insights into linear belts of high strain and high grade meta- the petrology and tectonic history of the major morphism, ophiolites — especially at higher components of the orogen and into both the crustal levels — clockwise PTt paths, arc magma- location and geodynamic evolution of the major tism and associated juvenile isotopic signatures crustal boundaries. This paper focuses on the core both in magmatic and sedimentary protoliths. Be- zone of the orogen — especially the Tersk, Lap- side their importance in understanding litho- land Granulite and Umba Granulite terranes — spheric history and architecture, suture zones can and emphasises evidence for large scale crustal facilitate large-scale correlation of orogenic belts. separation and growth of new crust before colli- Deeply eroded collisional sutures may lack ophio- sion c 1.9 Ga ago. lites, one of the most reliable criteria in their Balagansky et al. (1998a) has divided the LKO recognition. In such cases, suture zones may be into dispersed and accreted terranes. The dis- identified due to the presence of belts of subduc- persed terranes (Murmansk, Central Kola, Inari tion-related juvenile crust identified, e.g. using and Belomorian, Fig. 1) comprise fragments of a isotope geochemical data — especially SmNd rifted Neoarchaean craton, reassembled in the data combined with independent reliable UPb Palaeoproterozoic. The accreted terranes include mineral geochronology. Distinguishing true suture the Lapland Granulite Terrane (well known as the zones that continue to mantle depths from belts of Lapland Granulite Belt, LGB), Umba Granulite juvenile crust that are merely superficial al- Terrane, and according to recent data by Daly et lochthons requires deep geophysical images. Re- al. (1999), Tersk Terrane, all composed of cent reprocessing and interpretation of deep Palaeoproterozoic juvenile crust generated in an seismic refraction data (Pilipenko et al., 1999) island-arc setting (Huhma and Merila¨inen, 1991; suggest that this situation obtains within the Daly et al., 1997; Balagansky et al., 1998b). Palaeoproterozoic Lapland–Kola Orogen (LKO) These three terranes, together with the Tanaelv in the northern Fennoscandian Shield. This paper and Kolvitsa belts, make up the NW-trending presents new geochronological and isotope geo- core of the LKO between the Belomorian com- chemical data bearing on the location of the posite terrane and the Inari and Central Kola Lapland–Kola Suture (LKS) zone and on the composite terranes (Fig. 1). Collisional deforma- evolution of the core zone of the LKO (Marker et tion is strongly developed in the orogenic core al., 1993). and also extends southwards beyond the Belomo- rian into the Karelian composite terrane, e.g. in 1.1. Lapland–Kola Orogen the Kukas–Chelozero shear zone (Balagansky, 1992). From c 2.50 Ga onwards, the Archaean Long regarded as an Archaean craton, recent crust of the shield was extensively rifted and investigations have shown that the LKO is a partly dispersed. In the orogen core, rift magma- collisional orogen comprising mainly Archaean tism led to the formation of the Kolvitsa Belt, terranes finally welded together in the mafic dykes and anorthositic gabbro accompanied Palaeoproterozoic. Recent models for the devel- by transtensional shearing and metamorphism opment of the northern Fennoscandian Shield (Balagansky et al., 2000). High-P, high-T meta- (e.g. Gorbatschev and Bogdanova, 1993; Hjelt et morphism in the core and footwall of the orogen al., 1996) have emphasised the importance of is commonly attributed to c 1.9 Ga collision (Priy- Palaeoproterozoic collisional orogenic events atkina and Sharkov, 1979; Barbey et al., 1984), within the LKO while Bridgwater et al. (1992) manifested, e.g. by 1.90–1.92 Ga old metamor- J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 291 phic zircons in the Kolvitsa gabbro-anorthosite within the Tersk Terrane and Strelna Domain of massif (Frisch et al., 1995), dioritic dykes the Central Kola Terrane. SmNd whole-rock (Kaulina, 1996) and sillimanite–garnet–biotite analyses are used to identify an important compo- gneisses (Bibikova et al., 1973). nent of juvenile crust within the Tersk, Lapland Granulite and Umba Granulite terranes. SmNd 1.2. Aims of this paper and ArAr mineral ages are used to determine the timing of post-collisional metamorphism and sub- This paper aims to present a tectonic model for sequent cooling. These results are used to develop the Palaeoproterozoic LKO on the Kola Penin- a tectonic model for the evolution of the LKO, sula based on new geochronological and isotopic taking account of new and existing isotopic data evidence. Ion microprobe UPb zircon analyses and recent reinterpretation of deep seismic data. are used to determine the age of the main pro- Isotopic data are presented from three areas toliths and the timing of accretionary deformation (Fig. 1) — from a north–south section across the Fig. 1. Major tectonic divisions of the Kola Peninsula and adjacent areas. Terrane terminology after Balagansky et al. (1998a). Boxes show locations of Figs. 2–4. LKS, footwall boundary of the Lapland–Kola Suture. MR, Main Ridge gabbro anorthosite. Inset map shows the major divisions of the Fennoscandian (Baltic) Shield after Hjelt et al. (1996). L-K, Lapland–Kola Orogen; Ka, Karelia; Cal, Caledonian Belt; Svn, Sveconorwegian. 292 J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 Fig. 2. Sketch geological map showing the major tectonic divisions, sample localities and geochronological data from the Varzuga River section, southern Kola Peninsula. Ages (with 2s errors) are given in Ma, unless otherwise specified. LKO along the Varzuga River to the east, from (Fig. 2) across the Central Kola and Tersk ter- the Lapland Granulite Terrane at the northwest- ranes in an otherwise rather poorly exposed, yet ern end of the orogen and from its southeastern critical, part of LKO. correlative, the Umba Granulite Terrane on the From north to south, the Varzuga River White Sea coast. exposes: 1. the Imandra–Varzuga Sequence, a rift 1.3. Tersk Terrane and Strelna Domain zone which developed between c 2.5– 1.8 Ga (Zagorodny et al., 1982; Melezhik The Varzuga River in the southern Kola and Sturt, 1994; Mitrofanov et al., Peninsula provides an almost complete section 1995a). J.S. Daly et al. / Precambrian Research 105 (2001) 289–314 293 2. Neoarchaean tonalite–trondjemite–granodior- garnet–biotite gneisses and meta-anorthosites. ite (TTG) gneisses of the Strelna Domain These rocks in turn are thrust over migmatitic which form the Archaean basement to the TTG gneisses of the Belomorian Terrane.